Swift Long Jump Wrapper

Swift Long Jump Wrapper.swift

// We will show how long jumps can be used within Swift

// Create jump and set it's position
let start = Jump()
setjmp(start.line) // Sets start's line to the current line

println("HI") // -> "Hi" -> "Hi" -> "Hi" -> ... (prints forever)

longjmp(start.line, 0) // Jumps back to start's line, e.g. setjmp(start.line)

// We have created an infinite loop using jumps!
// Now let's do something more useful: We will create a while loop!

func myWhile(test: () -> Bool, loop:() -> ()){
    let start = Jump()
    setjmp(start.line)
    
    if (test()){
        loop()
        longjmp(start.line, 0)
    }
}

// Now we will use our loop!

var x = 0
myWhile({ x < 3 }){
    println("Hello world") // -> "Hello world"
    x++                    //    "Hello world"
}                          //    "Hello world"

// Let's look at a more advanced example
// It turns our that setjmp actually returns whatever value is passed into longjmp
// (unless the value 0 is passed in in which case setjmp returns 1)
// (and setjmp returns 0 the first time it is called)
// Note that setjmp and longjmp use Int32, so cast accordingly

func doNTimes(n: Int, loop:() -> ()){
    let start = Jump()
    let count = Int(setjmp(start.line))
    
    if (count < n){
        loop()
        longjmp(start.line, count + 1)
    }
}

doNTimes(3){
    println("Whoa") // -> "Whoa"
}                   //    "Whoa"
                    //    "Whoa"

// One more thing: Let's modify myWhile to support continue and break calls!

func myWhile(test: () -> Bool, loop:(myContinue: () -> (), myBreak: () -> ()) -> ()) {
    let start = Jump()
    let stop = Int(setjmp(start.line))

    if (stop != -1 && test()){
        loop({ longjmp(start.line, 0) }, { longjmp(start.line, -1) })
        longjmp(start.line, 0)
    }
}

// Tahdah!!

var x = 0
myWhile({ true }) { myContinue, myBreak in
    x++

    println("Considering \(x)")
    if x % 2 == 0 { myContinue() } // skip even numbers
    
    println("Using \(x)")
    
    if x > 9 { myBreak() } // print numbers less than or equal to 11
}

// Now for the actual implementation
// Turns out that I didn't really do much to make this possible
// C already has this feature built in, so naturally Swift does too
// I just wrapped up the malloc and free calls so you don't have to worry

class Jump {
    typealias JumpType = UnsafeMutablePointer<Int32>
    private let jumpSize = UInt(sizeof(jmp_buf))
    let line: JumpType

    init() {
        line = JumpType(malloc(jumpSize))
    }
    deinit {
        free(line)
    }
}